1. Field of the Invention
The present invention relates to a slab type laser apparatus for emitting a pulse laser beam, and particularly to a slab type carbon dioxide gas (CO2) laser apparatus for emitting a driver laser beam to be applied to a target material to turn the target material into plasma for radiating extreme ultraviolet (EUV) light in an extreme ultraviolet light source apparatus.
2. Description of a Related Art
Recent years, as semiconductor processes become finer, photolithography has been making rapid progress to finer fabrication. In the next generation, micro-fabrication at 60 nm to 45 nm, further, micro-fabrication at 32 nm and beyond will be required. Accordingly, in order to fulfill the requirement for micro-fabrication at 32 nm and beyond, for example, exposure equipment is expected to be developed by combining an extreme ultraviolet (EUV) light source generating EUV light having a wavelength of about 13 nm and reduced projection reflective optics.
Recently, an immersion method using an ArF laser enables even a micro-fabrication at 45 nm, and a micro-fabrication at 32 nm to 22 nm and beyond using the EUV light is going to be carried out in EUV lithography.
As the EUV light source, there are three kinds of light sources, which include an LPP (laser produced plasma) light source using plasma generated by applying a laser beam to a target, a DPP (discharge produced plasma) light source using plasma generated by discharge, and an SR (synchrotron radiation) light source using orbital radiation. Among them, the LPP light source has advantages that extremely high intensity close to black body radiation can be obtained because plasma density can be made considerably larger, that the light emission of only the necessary waveband can be performed by selecting the target material, and that an extremely large collection solid angle of 2π steradian to 4π steradian can be ensured because it is a point light source having substantially isotropic angle distribution and there is no structural member surrounding the light source such as electrodes. Therefore, the LPP light source is considered to be predominant as a light source for EUV lithography, which requires power of 100 W or more.
FIG. 31 shows a system configuration of an LPP type EUV light source.
An EUV light source generates plasma by focusing a driver laser beam onto a target material, such as tin (Sn), that is present in a vacuum chamber. A variety of wavelength components containing extreme ultraviolet (EUV) light are radiated from the generated plasma, and the desired EUV component among the variety of wavelength components is selectively reflected and collected by using a collector mirror (EUV collector mirror), and outputted to a device, such as an exposure unit, using the EUV light.
The EUV light source is required to have output power exceeding 100 W. For example, a carbon dioxide gas (CO2) laser beam generates EUV light with comparatively high efficiency, by being applied to tin (Sn) to generate plasma. Even a CO2 laser apparatus, which emits a driver laser beam for plasma generation, requires higher efficiency and space-saving for industrial applications.
FIG. 32 is a schematic diagram showing configuration of a master oscillator power amplifier type laser apparatus to be used as a driver laser.
A pulse laser is used for plasma generation. A master oscillator power amplifier type laser apparatus as shown in FIG. 32 includes a laser oscillator which is a short-pulse CO2 laser generator, and a laser amplifier for amplifying a short-pulse CO2 laser beam. The laser amplifier includes a discharge device for exciting a CO2 laser gas containing carbon dioxide (CO2), nitrogen (N2), helium (He), further according to need, hydrogen (H2), carbon monoxide (CO), xenon (Xe) and so on by discharge.
A laser beam (seed laser beam) having energy “A” emitted from the laser oscillator is amplified to be a laser beam having desired energy “B” in the laser amplifier. The laser beam having the energy “B” is focused through a laser beam propagation system or a lens, and is applied to an EUV light generation target material that is selected from tin (Sn), xenon (Xe) and so on.
The inventors of this application already proposed a driver laser for an extreme ultraviolet light source apparatus in Japanese Unexamined Patent Application Publication JP-P2009-26854A. The driver laser employs, as a laser amplifier for amplifying laser energy “A” to laser energy “B”, a slab type laser amplifier including an optical system having a plurality of mirrors for multi-path-reflecting a laser beam incident from a first window to propagate it in a discharge region and outputting it from a second window.
According to the proposed driver laser apparatus, since such a multi-path method is adopted such that an incident laser beam goes and returns plural times in a laser medium, it is possible to downsize a driver laser and amplify a laser beam efficiently.
However, the master oscillator power amplifier type laser apparatus having the above-mentioned configuration has an issue that a large installation area is required because the laser apparatus includes at least two laser modules such as an oscillator and an amplifier.
As a related technology, a fast axial flow CO2 laser amplifier is disclosed in an article, i.e. Tatsuya Ariga and Akira Endo “CO2 Laser for Extreme Ultraviolet (EUV) Rays Lithography Light Source”, O plus E, New Technology Communications, Volume 28, No. 12 (December 2004), pp. 1263-1267.